# Solar Attic Fans vs Wired vs None

#### shurguywutt

Main objective is to help cool the house and decrease load on the A/C during the hot season.

Agendas aside, when I look at the numbers they tell the truth. Solar fans get about 300-500 CFM (maybe). Hard wired get 1200(+) CFM. I could also do gable mount fans. The price of solar is much higher, I don't know if I could offset the higher price with power saving. I mean how much power do fans actually use? Then I started thinking....

Lets put our math hats on...

1.65A x 120V = 198W, lets just say 200W per fan.

So if I got 3 fans = 600W

My power company charges ~\$0.15 per 1KWH (1 hour use of 1000 watts).

So if 1000W = \$0.15, then 600W = \$0.15 X 0.6KWH = \$.09

I am probably looking at less than \$0.09/day to run them. I am sure they would run less than 1 hour a day even during the hottest season.

\$0.09 x 270 days = \$24.30 rough yearly operating cost. Not 365 days because I am also accounting for winter.

How many years to offset the higher price of solar fans?

Solar fans were quoted to be several thousand dollars...

I also could just leave the static ventilation.

I am old school. Show me the data kind of guy. Similar to most BITOG members. Would love to hear your thoughts.

Every home in my SoCal neighborhood has at least 2 on the home and 1 on the garage of these or similar:

At least half have solar panels, at least 25% have battery storage.

No need to have powered fans, let the wind do the work.

You must have sufficient soffit ventilation.

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I would go with hard wired fans. Initial cost of solar is too much and the ROI is too many years out into the future to break even.

Fan-Powered Ventilation
Although solar-powered or plug-in vent fans can be used to increase attic ventilation, and often include a thermostat that limits operation to the hottest part of the day. However, natural ventilation can achieve similar attic heat-flow reduction. Additionally, power ventilators can significantly depressurize the attic, pulling air from the house through small holes in the ceiling plane.

If you add a powered fan and it pulls any air from the house you are hurting your cooling performance. This is due to extra hot air being pulled into the conditioned area from the fan so I'd be careful that does not happen.

Not that this changes your conclusion but I would think the fan would run for longer than an hour unless you are setting the temp pretty high or your attic is well ventilated as is.

Also it seems these companies charging thousands for the solar fans are making serious margins. They costs about \$300 more than the wired fans at Home Depot and the installation process is the same or easier since you don’t have to run any wiring.

I had a solar fan installed in this sort of side attic area that wasn’t getting good airflow because of the roof shape. I had some roof repair guys install it for much cheaper than the companies marketing the solar fans for energy savings.

Every home in my SoCal neighborhood has at least 2 on the home and 1 on the garage of these or similar:

At least half have solar panels, at least 25% have battery storage.

No need to have powered fans, let the wind do the work.

You must have sufficient soffit ventilation.
I do like those but unfortunately our area gets high winds and those are not a good option.

I would go with hard wired fans. Initial cost of solar is too much and the ROI is too many years out into the future to break even.
My thoughts exactly. Solar loses efficiency over the years as well.

If you add a powered fan and it pulls any air from the house you are hurting your cooling performance. This is due to extra hot air being pulled into the conditioned area from the fan so I'd be careful that does not happen.
Right, I have soffit vents around the exterior of the house. I don't think this would be a problem.

Would love to hear your thoughts.

Not that this changes your conclusion but I would think the fan would run for longer than an hour unless you are setting the temp pretty high or your attic is well ventilated as is.

Also it seems these companies charging thousands for the solar fans are making serious margins. They costs about \$300 more than the wired fans at Home Depot and the installation process is the same or easier since you don’t have to run any wiring.

I had a solar fan installed in this sort of side attic area that wasn’t getting good airflow because of the roof shape. I had some roof repair guys install it for much cheaper than the companies marketing the solar fans for energy savings.
Yes, they are making huge margins. If the fans stay on too long I can adjust the thermostat. Manufacturer recommends 105* for best efficiency. As an aside note, I know there are some solar units that are very cheap. Which doesn't really make sense to me because I know decent solar panels are much more than \$300.

Seems like passive works fine for most people

Yes it is
Then you are wasting money looking at attic ventilation. It will not lower the AC load, it will just add to your electric bill that’s about it.

You also mentioned high wind in your area, that alone helps ventilating the attic.

I would look into topping off the attic insulation and that’s about it.

Then you are wasting money looking at attic ventilation. It will not lower the AC load, it will just add to your electric bill that’s about it.

You also mentioned high wind in your area, that alone helps ventilating the attic.

I would look into topping off the attic insulation and that’s about it.
So if my home had poor insulation then I should look into attic ventilation?

Main objective is to help cool the house and decrease load on the A/C during the hot season.

Agendas aside, when I look at the numbers they tell the truth. Solar fans get about 300-500 CFM (maybe). Hard wired get 1200(+) CFM. I could also do gable mount fans. The price of solar is much higher, I don't know if I could offset the higher price with power saving. I mean how much power do fans actually use? Then I started thinking....

Lets put our math hats on...

1.65A x 120V = 198W, lets just say 200W per fan.

So if I got 3 fans = 600W

My power company charges ~\$0.15 per 1KWH (1 hour use of 1000 watts).

So if 1000W = \$0.15, then 600W = \$0.15 X 0.6KWH = \$.09

I am probably looking at less than \$0.09/day to run them. I am sure they would run less than 1 hour a day even during the hottest season.

\$0.09 x 270 days = \$24.30 rough yearly operating cost. Not 365 days because I am also accounting for winter.

How many years to offset the higher price of solar fans?

Solar fans were quoted to be several thousand dollars...

I also could just leave the static ventilation.

I am old school. Show me the data kind of guy. Similar to most BITOG members. Would love to hear your thoughts.

Vented attics for all intensive purposes are considered "outdoor space". It's covered, but still open to the outdoors.

For your hot-humid climate an attic fan will.

- Pull conditioned air from the lower floor into the attic. Consequently it will suck in hot humid air from the outside through cracks in the building envelope and through doors/windows. This also wastes energy.

A better option

-Air seal the ceiling below the attic and add more insulation. This means sealing around HVAC registers, light fixtures, bath vents, and attic hatches. It's not expensive to do.

- If your A/C system is in the attic then that energy penalty is going to be difficult to overcome without spending some \$\$\$ to create a hot roof (see *). At the very least you least want to make sure your duct work in the attic isn't leaking because if it is then you're spending \$\$ to air condition the outdoors. An added feature of leaking attic HVAC ducts is that it allows the system to suck in humid air from the outside via returns ducts on the lower floors.

**Hot Roof - Basically you re expanding the interior of the house from the ceiling to the roof.
You can do that in two ways:
No 1 - Spray the interior side of the roof deck with closed cell spray foam (ccSPF). I wouldn't use open cell in Florida unless you already have a ridge vent which you'd want to keep.
No 2 - Install rigid foam boards against the exterior of the roof decking and install shingles/tiles on top of it.

You also want to add a small supply duct to the attic to help with dehumidification. Water vapor always travels into the attic space so you want to be able to deal with that.

A hot roof also allows you to get rid of ceiling insulation.

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An issue I didn't see mentioned is the quality of your typical residential grade roof fan. They're terrible. It's not uncommon to have the motors fail in them in 2yrs. If they're in a spot where they're not easy to get it, they wind up just another box vent.

Heat rises so why do you think hot attic air will somehow fall downwards into the living space? Especially if there is enough insulation above the ceiling. Those fans may result just a minimal net reduction in your energy bill, similar to what a water heater blanket offers. It's more a, feel good save the planet thing.

I do like those but unfortunately our area gets high winds and those are not a good option.
If those are wind powered, isn't that the perfect option? Uses no power at all.

An issue I didn't see mentioned is the quality of your typical residential grade roof fan. They're terrible. It's not uncommon to have the motors fail in them in 2yrs. If they're in a spot where they're not easy to get it, they wind up just another box vent.
This is also a consideration. I don't want to be going up there ever 2 years replacing Chinazon motors. What a pain in the arse.
Heat rises so why do you think hot attic air will somehow fall downwards into the living space? Especially if there is enough insulation above the ceiling. Those fans may result just a minimal net reduction in your energy bill, similar to what a water heater blanket offers. It's more a, feel good save the planet thing.

If those are wind powered, isn't that the perfect option? Uses no power at all.
Not 100 mph winds aka. hurricanes. You have to remove them when those roll through.

I do like those but unfortunately our area gets high winds and those are not a good option.

Unless it's a hurricane I don't see them having a wind rating and in that case your roof is gone anyway.

I would suggest if you really think your attic fan only needs to run an hour then you dont need one.
I used to have a mid output electric attic fan in a northern climate. I think set to come on around 110 degrees, (grabbing at straws with that number) and it would run from about 3Pm to 9ish pm in the summertime.

I wouldnt get a solar unless it was of the ultimate highest quality and I am not sure where that would come from.
You dont mention the size of your house, roof surface and other criteria but 3 fans sounds VERY excessive way more so if you think it's not going to run more than an hour a day. One fan should do it. Personally I think you are more in danger of drawing outside air into the home and if you have any fossil fuel vents into the outside you may draw combustion gases back into the house.

Other factors are how is the attic vented now? I know you have soffits but ...
Is it vented with gables on each side or are they ridge vents along the top of the entire roof line?
If its ridge vents the efficiency of the fans will be greatly reduced as air will flow from the least resistance. With a ridge vent air will be pulled from the same area of the roof as the fan vs pulling the hot air out and drawing air from the gables across the attic as well as from the soffits.

I would think the smartest avenue is to get a indoor/outdoor thermometer, one that stores the highest temperature of the day and put the sensor in the attic to understand just how hot the attic is getting.

Once you get all that figured out then if you proceed with a fan make sure it is of high quality.

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